1. Field of the Invention
This invention relates to low emissivity coatings on transparent substrates and, in particlar, to low emissivity silver coatings comprising a layer of silver and an overlying anti-reflective layer of metal oxide, and to the production of such coatings.
2. Description of the Prior Art
Low emissivity silver coatings are known and have been described in the prior art, for example in U.K. Patent Specification 1,307,642. This specification describes electrically conductive glass articles comprising a glass substrate and an electroconductive coating and specifies the use of an intermediate layer of not less than 50% silver 200 to 300 A thick to provide an electrical resistivity of not more than 3 ohms/square disposed between a pair of layers of non-absorbing dielectric material as anti-reflection layers, each 70 to 550 angstroms thick, to increase the light transmission of the coated glass. The specification proposes incorporating up to 10% of chromium, nickel, aluminium or titanium, or up to 50% copper, in the silver layer; use of copper is said to provide a transmission colour of grey which, according to the specification, cannot be easily obtained with a film composed substantially of silver. It is said that the deposition of silver or metal oxide may be conducted by cathodic sputtering. To form a silver layer incorporating an additional metal, either a silver alloy is evaporated or the metal elements are simultaneously evaporated under vacuum.
U.S. Pat. No. 4,166,876 describes a coating comprising a layer of metal such as silver, gold, copper, platinum or tin sandwiched between two layers of a titanium oxide on a plastics substrate. The patent teaches that, if the lower layer of titanium oxide is derived from an organic titanium compound and contains residual organic moieties, the bond to the resin substrate is markedly improved with an improvement in the transparency of the laminated structure. The specification teaches that the silver layer may contain 1 to 30% of copper which reduces the tendency of the coating to degrade, and gradually lose its light reflecting property, on prolonged exposure to light; the copper-containing silver layer may be deposited by vacuum deposition from a silver-copper alloy.
European patent specification No. 0 035 906 describes a coating comprising a layer of silver sandwiched between two layers of metal oxide. The metal oxide layers may be deposited by sputtering, ion plating, vacuum deposition or from solution. The patent teaches that a thin layer of material selected from the group consisting of titanium, zirconium, silicon, indium, carbon, cobalt and nickel should be deposited between the silver and the overlying metal oxide layer to improve the long term durability of the coating. The specification teaches that the material should be deposited under conditions such that, as far as possible, it is not converted to an oxide; and, where an overlying metal oxide layer is deposited by sputtering, the sputtering is carried out using an oxide source under an argon atmosphere thereby avoiding as far as possible oxidation of the material.
Silver coatings of the kind described above i.e. consisting of silver layers sandwiched between anti-reflection metal oxide layers not only have a high conductivity, but also exhibit a low emissivity i.e. they reflect a high proportion of infra-red radiation incident upon them whilst allowing short-wave infra-red radiation and visible radiation to pass through. The use of such coatings on window glass (or plastics used in place of glass) leads to a reduction in heat loss from the windows and, with increasing energy costs, is becoming increasingly desirable in order to reduce heating costs. Unfortunately, when attempts were made to produce a coating comprising a metal oxide layer on top of a silver layer by a reactive sputtering process in the presence of oxygen, it was found that the low emissivity properties of the silver layer were lost, and the light transmission of the product was substantially lower than expected.